PROJECT SUMMARY/ABSTRACT In the United States and worldwide, the leading causes of mortality include substance use disorders and risky sexual behaviors that contribute to sexually transmitted diseases, including human immunodeficiency virus (HIV) infection (NIDA, 2010; CDC, 2011, 2012; WHO, 2008). Negative peer influence is a major risk factor for early initiation of substance use and other risky behaviors (Pfeifer et al., 2011). However, group- based interventions and positive social supports are also known to be beneficial for successful cessation of substance use and addictive behaviors (Hayes et al., 2006; Ouimette et al., 1997; Simoni et al., 2007). Thus, social influences contribute differentially depend on the context and nature of the influence. Yet, the mechanisms underlying susceptibility to social influence have received limited investigation. Here, we combine functional neuroimaging and variants of a novel decision-making paradigm to examine how individuals with cocaine use disorder (CUD) process and use social influence when making decisions about risky options. Our broad hypothesis is that disrupted neurobehavioral processing of information from social others contributes to the maintenance of substance use disorders and may explain why risky others have an unusually large impact on those who engage in substance use. We combine functional neuroimaging and a model-based behavioral economics approach to test this hypothesis in CUD participants. Specifically, in Aim 1, we test the possibility that CUD individuals value information from risky others more than information from safe others, thus contributing to increased likelihood to engage in risky behaviors. In Aim 2, we test the possibility that the source of information about risky options guides decision-making such that voluntarily sought information from social others is more influential to decision-making than is information that is externally provided (and involuntarily acquired). In both Aims, we propose to scan CUD individuals as they make decisions about risky options alone and within a group and test the influence of others on subjects' decisions. This general approach allows us to examine the neurobehavioral processes associated with both the beneficial and detrimental effects of social influence on risky decision-making and thus has implications for understanding both the onset and prevention of disordered substance use.